Wireless communication systems exist in various forms. Typically, a wireless communication system could include one or more wireless terminals and one or more base stations. In a cellular system for wireless communication, each wireless terminal in the system is connected to one or more base stations via radio communication. The base stations can be part of a serving network (NW), which could also include other entities for controlling the traffic in the system, such as e.g. a Radio Network Controller (RNC). Transmission from a wireless terminal to a base station is called uplink (UL) transmission, and transmission from a base station to a wireless terminal is denoted downlink (DL) transmission.
In a wireless communication system, different multiple access techniques could be used—alone or in combination—to enable several users to communicate with the serving network simultaneously. Throughout this description, reference will be made to Code Division Multiple Access (CDMA) systems. In contrast to Time Division Multiple Access (TDMA) systems and Frequency Division Multiple Access (FDMA) systems, where users are separated in time and frequency, respectively, all active users in a CDMA system typically share time and frequency resources. To separate the users in a CDMA system, each user employs a specific code (often referred to as a spreading code) that distinguishes that particular user from the other users.
There exist two different principles regarding how to apply the user specific code in CDMA. In Frequency Hopping-CDMA (FH-CDMA), the code is used to define a sequence of frequencies. These frequencies are sequentially used for transmission, each frequency being applied a relatively short period of time. In Direct Sequence-CDMA (DS-CDMA) the signal to be transmitted is multiplied by the spreading code before transmission. Both FH-CDMA and DS-CDMA are well known in the art.
In a DS-CDMA system, a spreading code can be built up in several steps, e.g. by first applying a user specific code that is orthogonal to all other user specific codes used by that base station, and then applying a base station specific pseudo-noise sequence which accomplishes a frequency spreading of the signal. The symbols that constitute the spreading code (typically +/−1) are usually referred to as chips.
One particular example of a CDMA system is Wideband CDMA (WCDMA). WCDMA is used in Universal Mobile Telecommunications System (UMTS), which is advocated by the 3rd Generation Partnership Project (3GPP). Without loss of generality, notations from UMTS/WCDMA will be used in this description. In UMTS, a base station is often referred to as Node B, and a wireless terminal is referred to as a User Equipment (UE).
Examples will also be presented by referring to logical and physical channels as specified in WCDMA, such as the Dedicated Control CHannel (DCCH) which is a logical channel used to transmit user specific control signalling and which is mapped onto the Dedicated Physical Data CHannel (DPDCH); the Dedicated Physical Control CHannel (DPCCH) which is a physical channel comprising e.g. Transmit Power Control (TPC) symbols, Transport Format Combination Indicator (TFCI) bits, etc; the Dedicated Physical CHannel (DPCH) which is a user specific physical channel intended for traffic data transmission; the Broadcast CHannel (BCH) which is a physical channel used to broadcast information intended for all users; and the Common Pilot CHannel (CPICH) which is a physical channel used to transmit pilot signals that can be used for e.g. channel estimation since these pilot signals are common for, and known by, all entities in the system.
Other channels that will be referred to are defined in connection to High Speed Downlink Packet Access (HSDPA), which is one of the services available in UMTS. Examples of HSDPA channels are the High Speed-Physical Downlink Shared CHannel (HS-PDSCH) which is a physical channel used to transmit high speed DL packet data, and the High Speed-Shared Control CHannel (HS-SCCH) which is a physical channel used for DL control signalling. The UE uses the information on the HS-SCCH to determine whether data packets transmitted on the HS-PDSCH are intended for that particular UE.
All references and notations from WCDMA are merely examples, meant to make this description more illustrative, and should by no means be construed as limiting or restrictive. On the contrary, this invention—as defined by the appended claims—can be practised in various other ways and can be applied to other standards than WCDMA, as can be understood by a skilled person.
The developments within the field of wireless communication have resulted in the evolution of increasingly advanced receivers. One of the benefits of such advanced receivers might be that they can offer better receiver performance than less advanced receivers. Comparing an advanced receiver with a less advanced receiver, this better receiver performance could, for example, manifest itself as a lower decoding error probability for the advanced receiver when both receivers experience the same received signal quality. Looking at it from another perspective: if both receivers are to have the same decoding error probability, the advanced receiver accomplishes that while experiencing worse received signal quality than the less advanced receiver.
One might use e.g. the signal-to-interference ratio (SIR) as a measure of the quality of a received signal, and e.g. bit error rate (BER) or block error rate (BLER) as a measure of the quality of the demodulated symbols after processing by a receiver. If, for example, the SIR of a signal received at a wireless terminal is decreased, the BER of the demodulated symbols after processing by an advanced receiver is still similar to, or better than, if the SIR of the received signal was not decreased and a less advanced receiver was used.
Consequently, the required quality of the received signal can be decreased for wireless terminals using advanced receivers. This might improve system capacity e.g. by allowing reduction of the transmission power used at the base station for transmissions to the wireless terminal. The same is true for the reversed situation; when a base station uses an advanced receiver, the wireless terminals transmitting to that base station could lower their transmission power.
Examples of advanced receivers are: an interference-cancelling receiver, such as a CPICH interference-cancelling receiver; a generalized RAKE (G-RAKE) receiver as described in for example EP 1197007 B1; variants of a G-RAKE receiver; and a chip equalizer.
Typically, advanced receivers experience longer processing time for steps performed by the receiver than a relatively less advanced receiver, such as a conventional RAKE receiver, does. For example, a G-RAKE receiver generates combining weights based on channel estimates and on an estimated interference correlation matrix. In particular the generation of the interference correlation matrix requires quite a large amount of baseband processing, which is performed for example in a digital signal processor (DSP). This results in a prolonged processing time compared to the processing time of a conventional RAKE receiver, since the RAKE receiver does not generate the estimated interference correlation matrix. Thus, in this context, an advanced receiver could be seen as a receiver having better performance quality, but longer processing time than a less advanced receiver.
This extra processing time might not pose any problems to the timing requirements of most portions of the signalling, such as e.g. the traffic information data symbols of the DPCH. For such portions of the signalling, symbols can be stored in a memory until the baseband processing of the advanced receiver, such as for example the calculation of the interference correlation matrix and the combining weights for a G-RAKE receiver, have been completed.
However, there might be certain portions of the signalling that cannot apply an advanced receiver because the delay due to the extra processing time is not acceptable. For example, the demodulation of certain portions of the signalling might have to answer to tighter real time requirements to be able to comply with a wireless communication standard such as the 3GPP standard. If an advanced receiver was applied to those certain portions of the signalling, the receiver might not be able to produce the demodulated symbols during a time frame that corresponds to the timing requirements. Examples of such certain portions of the signalling are the transmit power control (TPC) commands, the feedback information (FBI) commands, and the HS-SCCH as defined in the 3GPP standard.
For simplicity of notation, those certain portions of the signalling that are processed by the less advanced receiver will be referred to herein as time critical control signals (TCCS), and the signalling that does apply an advanced receiver will be referred to as non-TCCS. In the examples described above, the reason for using the less advanced receiver for the TCCS is that the TCCS cannot apply an advanced receiver because the delay due to the extra processing time is not acceptable. It should be understood though, that the invention is applicable to any situations where an advanced receiver is used to process a first portion of the received signal, and a less advanced receiver is used to process a second portion of the received signal. The reason for using the less advanced receiver for part of the processing is not necessarily that the delay due to the extra processing time of the advanced receiver is not acceptable.
TPC commands are used to control transmission power in both UL and DL of WCDMA systems. For power control of the UL, the base station evaluates the quality of the signals it receives from each UE compared to the quality requirements as defined in the WCDMA standard. Based on the evaluation, the base station determines whether a better or worse quality of the received signal is desired for each UE. A TPC command indicative of the desired change in quality is transmitted to each UE, which may or may not adjust its transmission power accordingly. A similar scenario exists for power control of the DL, where each UE evaluates the quality of the signal it receives from the base station compared to the quality requirements as defined in the WCDMA standard. Based on the evaluation, each UE determines whether a better or worse quality of the received signal is desired. A TPC command indicative of the desired change in quality is transmitted to the base station, which may or may not adjust its transmission power to that particular UE accordingly.
The evaluation and determination described above is often performed by a two-step control algorithm. A fast inner control loop, with e.g. a reference SIR value and an estimated actual SIR value as inputs, is used to control the TPC command. A slower outer control loop, with e.g. a reference BLER value and an estimated actual BLER value as inputs, controls the reference SIR value. The control mechanisms could comprise any control algorithms known in the art, such as a PD- or PID-regulator. For example, the inner control loop could comprise determining whether the estimated actual SIR value is greater than the reference SIR value or not. Usually, the TPC command comprises an “up”-instruction if the estimated actual SIR value is too low compared to the reference SIR value, and a “down”-instruction if the estimated actual SIR value is too high compared to the reference SIR value.
FBI commands are used in systems that apply transmit antenna diversity, i.e. when a transmitter employs several antennas to transmit the same message. The diversity is obtained by using, among other things, different phases for transmissions from the two antennas. The receiver can then benefit from this diversity, resulting in a diversity gain, i.e. better performance. A condition for obtaining the diversity gain is that the receiver is aware of the phase difference applied by the transmitter. This could be achieved, for example, by letting the receiver instruct the transmitter about what phase difference to use. This is done in WCDMA closed loop mode 1 and 2 transmission diversity, where Node B comprises at least two antennas and the UE transmits FBI commands on the UL to instruct Node B about the desired phase difference.
Since the UE evaluates the phase difference of the signals after channel propagation, it may instruct the base station to use a phase difference that gives the largest diversity gain at the receiver. This is beneficial since the propagation channel may alter the phase difference of the signals from the diversity antennas. Hence the phase difference that is believed to render the largest diversity gain, if determined without knowledge about the propagation channel, might not be equal to the phase difference that actually renders the largest diversity gain.
The description now returns to why TCCS cannot apply an advanced receiver and exemplifies with TPC commands transmitted in the DL to control the UL. Those TPC commands need to be decoded promptly so that the UE can be able to adjust the UL power according to the timing requirements in the 3GPP standard. In the 3GPP standard (Appendix B of 3GPP TS 25.214) it is required that the UE responds to the TPC command by adjusting the UL power within 512 chips from the reception of the TPC command. This means that the time available for processing the TPC command is 512 chips minus the maximum delay spread of the channel, which typically equals 80-100 chips. Similar timing requirements exist for TPC commands transmitted in the UL to control the DL. For FBI commands there is no strict timing requirement in the 3GPP specification. However, Node B needs to adjust to the FBI commands as soon as possible for the system to work properly. Hence, advanced receivers, such as a G-RAKE receiver, cannot be used for the processing of TCCS, such as the TPC or FBI commands. Therefore, receivers comprising an advanced receiver typically have to use a less advanced receiver, such as a RAKE receiver, for the reception of TCCS.
If, for example, a UE with an advanced receiver was employed in a WCDMA system as it is designed today, it would be seen that the improved reception performance of the advanced receiver would lower the power of the DL to a level where the required quality of the demodulated symbols, e.g. required BLER, was still achieved. The performance gain of advanced receivers, i.e. how much the DL power can be reduced compared to if a less advanced receiver was used while still obtaining the same receiver performance, can be in the order of several dB.
However, since the TCCS cannot be processed by the advanced receiver, as explained above, the improved performance is not available for the TCCS. Hence, since the power of the DL is lowered, this will lead to a deteriorated quality of the processed TCCS. For example, for TPC commands the error rate might increase from 5% to 15-40% due to the decreased DL power. Such an increase of the TPC command decoding error rate will potentially destabilize the power control loops. If a majority of the transmitted TPC commands constitute a “down”-instruction and the receiver experiences a high TPC command decoding error rate, then an unnecessarily high UL power consumption results. If, even worse, a majority of the transmitted TPC commands constitute an “up”-instruction and the receiver experiences a high TPC command decoding error rate, then Node B could loose the synchronization with the UE and a dropped call could result.
The simulation results shown in FIG. 1 illustrate possible consequences of the deteriorated TPC decoding in a receiver comprising an advanced receiver. A channel case 1 where the channel consists of 2 taps, as defined in the 3GPP standard (“UE radio transmission and reception (FDD)”, 3GPP TS 25.101) has been assumed, and Îor/Ioc has been varied. Îor for is the total power spectral density of the DL transmitted from Node B as seen at the UE antenna connector, and Ioc is the power spectral density as measured at the UE antenna of a band limited white noise source, which is meant to simulate interference from cells which are not defined in a test procedure. Hence, Îor/Ioc represents a SIR value. UL and DL inner as well as outer power control were used in the simulations. Three different receivers have been examined: a conventional RAKE with 2 fingers (*), a G-RAKE with 2+2 fingers and one receiver antenna denoted G-RAKE 1 (+), and a G-RAKE with (2+2)*2 fingers and two receiver antennas denoted G-RAKE 2 (Δ). In the G-RAKE 2 case, the correlation between the antennas is assumed to be 0.7 and the gain for antenna 2 is −10 dB compared to antenna 1. A standard RAKE receiver is used for the TPC detection in all three cases. Furthermore, speech service with slot format 8 was assumed (spreading factor 128, 2 pilot symbols and 1 TPC symbol, as defined in 3GPP TS 25.211). The resulting average DL code power (i.e. Ec/Îor, the ratio of the average transmit energy per chip Ec to the total transmit power spectral density Îor) denoted by dashed lines, as well as the average UL output power denoted by unbroken lines can be found in FIG. 1 as functions of Îor/Ioc.
In the simulation results, an increase in the needed UL power due to a higher decoding error rate for the TPC commands can be clearly seen. A larger DL gain leads to a larger UL loss due to bad TPC detection performance. As mentioned above, this is due to the fact that the better the DPCH decoding is; the lower the received DL SIR needs to be. This in turn increases the TPC decoding error rate because the conventional RAKE detector, which is used for the TPC commands, now operates on a too low SIR, giving TPC decoding error rates of 15-40% (instead of 5%, which is a typical value appropriate for fulfilling the requirements in the standard).
As can be seen in FIG. 1 the average DL power reduction is large in the G-RAKE 1 case (for high Îor/Ioc) and very large in the G-RAKE 2 case (for all Îor/Ioc) compared to the RAKE case. However, the larger the gain is, in terms of DL power reduction, the larger the average loss of UL power becomes, as can be seen in FIG. 1, due to that erroneous TPC decoding increases the UL power variations. This could force Node B to increase its SIR reference value, and hence transmitting an excessive amount of “up”-instructions in order to maintain an acceptable quality of service. The result is decreased UL capacity. There is also a risk for dropped calls as described above, but not apparent from FIG. 1.
Similar situations may arise when the UL power is reduced due to that Node B employs an advanced receiver. For example, TPC commands are also employed to control transmission power of the DL as mentioned above, and problems very similar to those described above in connection to UL power control, may occur for DL power control. Another example of problems arising due to UL power reduction is provided when studying the FBI commands. As mentioned before, an advanced receiver is not feasible to use for the FBI commands due to timing requirements. Consequently, the detection of the FBI commands will suffer from a higher error rate if the UL power is reduced due to that Node B employs an advanced receiver. This in turn could result in that Node B applies an erroneous phase difference for the transmission from the diversity antennas. Hence, the diversity gain could be lost, which decreases the DL capacity of the system. The performance might even be worse than the performance of a system not employing transmit diversity at all.
In a UE adapted for HSDPA, the HS-PDSCH could with advantage be processed using an advanced receiver. However, the advanced receiver might not be applicable to the processing of the HS-SCCH, e.g. due to timing requirements. The HS-SCCH must be processed within a certain timeframe, otherwise the UE risks missing the data packets on the HS-PDSCH that were intended for that UE. Hence, a scenario similar to the ones described above arises. The power used to transmit the HS-PDSCH could be lowered since the HS-PDSCH is processed by an advanced receiver. This in turn deteriorates the quality of the determinations made based on HS-SCCH, since the HS-SCCH is not processed using the advanced receiver.
To conclude, wireless receivers that comprise an advanced receiver might experience a deteriorated error performance for the TCCS, for which the advanced receiver is not used. Therefore, there is a need for systems, methods and devices that obtain acceptable receiver performance quality of all portions of the received signal for each individual transceiver that comprises at least two receivers, and where the at least two receivers are used to process different portions of the received signal.
In the 3GPP specification TS 25.214 “Physical Layer Procedures (FDD)” it is disclosed that the network can set a power offset (PO2) to be applied to the TPC command portion of the signalling. This power offset will be applied equally to all UEs in the network irrespective of their individual needs.
In WO 02/23764 A2 the energy at which a transmit power control is transmitted is set based on how important it is that the transmit power control command is received. As an indication of how important it is that the transmit power control command is received, a difference between a measured quality, e.g., SIR, of a received signal and a reference may be determined.
In HSDPA Channel Quality Index (CQI) values are reported in the UL at regular time intervals. The reported CQI value is based on the SIR experienced by the receiver used to process HS-PDSCH in the UE. Based on the received CQI reports, Node B determines what power, code rate, and symbol alphabet to use for transmissions on the HS-PDSCH. Based on the CQI reports and different assumptions, Node B can also determine a power offset to be used for transmission of the HS-SCCH. That is, the HS-SCCH could be transmitted using a higher transmission power than the HS-PDSCH as determined by the power offset.